1. Field
The present disclosure generally relates to optical devices. More specifically, the present disclosure relates to an optical de-multiplexer (de-MUX) that includes an echelle grating having a cyclic free-spectral range (FSR).
2. Related Art
Integrated silicon photonics is a promising new technology that provides a number of advantages for chip-level communication, such as very high index contrast and compatibility with CMOS fabrication technology. Ongoing research into integrated silicon photonics is focusing on opportunities to provide low latency, high bandwidth, high density, and low power consumption. To date, several key active elements, including silicon lasers, modulators, and photodetectors, have been realized in silicon using low-cost CMOS compatible processes. However, a wavelength filter (such as an optical de-MUX), which can be used in wavelength division multiplexing (WDM), has not been developed yet.
In principle, a number of optical de-MUX designs can be integrated with CMOS circuits, including: Mach-Zehnder (MZ) lattice filters, ring resonators, arrayed waveguide gratings (AWG) and planar concave gratings (echelle gratings). Echelle gratings are particularly interesting because they are less sensitive to many fabrication errors, and they can be easily scaled up to large channel counts while still maintaining a very compact footprint.
However, echelle gratings are sensitive to non-vertical grating facets and facet roughness, which can result in optical crosstalk and on-chip insertion loss. One approach for solving these problems is to use a very small diffraction angle from an echelle grating. For example, by placing the receiving optical waveguides very close to the zero degree diffraction angle, light focused onto the output optical waveguides (which carry the higher-order diffraction modes) may have almost the same wavelength as the fundamental mode. Consequently, the optical crosstalk and the insertion loss can be insensitive to the vertical angle of the grating facet. A disadvantage of this approach is that, with a very small diffraction angle, for a given linear dispersion, the focal length for the Rowland circle on which the output optical waveguides are arranged (and hence the size of the device) increases dramatically. In particular, compared with echelle gratings with a typical diffraction angle, this arrangement of the output optical waveguides may increase the device size by a factor of 10, which increases the manufacturing cost and may limit applications in size-sensitive optical links.
Another approach to solving the problems of optical crosstalk and insertion loss is to reduce the silicon thickness in a silicon-on-insulator (SOI) technology that is used to implement an echelle grating in an optical de-MUX (for example, the silicon thickness may be reduced to submicron, which is sometimes referred to as ‘nano-photonic SOI’. In a nano-photonic SOI platform, dedicated deep etching techniques may not be needed because an improved grating profile can be obtained with less corner rounding, more vertical facets and significantly smoother sidewalls. In addition, the dependence of the optical crosstalk on facet verticality can be relaxed. For example, in order to maintain a 20 dB crosstalk performance, the maximum vertical tilt angle of the echelle grating can exceed 3° for a nano-photonic SOI platform, while for a thick SOI platform (with a silicon thickness of 2-5 μm), the echelle grating tilt may need to be tightly controlled to below 0.5°. Furthermore, when the silicon thickness is reduced to 0.25 μm, the free-propagation region becomes single mode. Therefore, there is no deterioration of optical crosstalk and insertion loss from coupling to higher modes.
However, in contrast with a thick SOI platform, the control of the center wavelength for an optical de-MUX implemented using a nano-photonic SOI platform can be extremely challenging because these devices are very sensitive to silicon thickness variation. For example a 3% variation in the silicon thickness is predicted to result in a central wavelength shift of approximately 10 nm for 0.25 μm SOI. As a consequence, wavelength tuning may be needed to correct for manufacturing-induced phase errors to align the optical de-MUX spectrum with transmitter channels, but this large tuning range may limit applications in energy-sensitive optical interconnects.
Hence, what is needed is an optical de-MUX that does not suffer from the above-described problems.